Improve ReadDOM properties using polyvariants

[dodomorandi]

I added some properties that could not be expressed correctly in ReasonML and added typing to others.
I just took the properties with meaningful comments about fixed states, therefore more work could be needed.

Just a small note: sandbox is quite tricky, because it should be a string containing space-separated fixed set of things. In practice we would need a list of an hypothetical Sandbox union type and it should be rendered as a joined string. I don't think that we have an easy way to express this, but if someone has an idea I will be happy to try something.

Closes #10

[rickyvetter]

Love this work, but we are moving all of this out of runtime into the ppx so the code that manages this will be in a different place. Landing this ahead of time would make it so we'd have two breaking changes so I don't think it's worth it.

Thanks for doing all this work and if you'd be interested I'd love to have you take a look at the ppx to make sure things are all in order and no support you were looking for is missing there. rescript-lang/syntax#235